Ion-sensitive sensors having an electrolyte insulator semiconductor structure (EIS structure), especially ion-sensitive field effect transistors (ISFETs), ion-sensitive, capacitive, respectively capacitively readable, sensors having an EIS structure or light operated, ion-sensitive LAPS sensors (LAPS=Light Addressable Potentiometric Sensor) having an EIS structure, are applied for measuring ion concentrations or special substance concentrations in solutions of different compositions and conductivities. Applications of ion-sensitive sensors with ISFETs, EIS elements and LAPS elements for the continuous checking of concentrations exist, for example, in the fields of environmental monitoring, industrial process monitoring, the foods industry and biochemistry, respectively medical technology. In such case, especially an as exact as possible concentration registration exhibiting an as small as possible, long term drift of the sensor element combined with an acceptable purchase price are desired for a corresponding ion-sensitive sensor.
Used as semiconductor material for the respective EIS structures, respectively EIS elements, is frequently silicon (Si), so that silicon dioxide (SiO2) is used as a first insulation layer. Other semiconductor/insulator combinations can frequently not deliver comparable properties as regards the required measurement accuracies during the required lifetime and are, furthermore, frequently not stably reproducible. Traditionally, ion concentration measurements in aqueous media are performed with glass electrodes both in process measurements technology as well as also in laboratory measurements technology. For technical reasons, especially because a sufficiently large inner buffer volume is required coupled with a sufficiently stable glass membrane, there is no opportunity for an efficient miniaturization of conventional glass electrodes. Moreover, it is to be noted that a pH-measuring system using a glass electrode has high impedance due to the needed glass membrane thickness and, thus, reacts sensitively to electrical environmental disturbances. This requires, among others, a shielding of the measuring lines, wherein the separation between the electrode and the measuring device should be as small as possible.
A further inherent disadvantage of the application of glass electrodes for pH-measuring is that, due to the application of the material, glass, there is a glass breakage risk under certain conditions, so that the use of glass electrodes in certain fields, such as e.g. foods technology, etc., is only limitedly possible.
For this reason, it is, consequently, attempted to use ion-sensitive EIS structures, respectively EIS elements, especially in the form of field effect transistors (ISFETs), for ion concentration measurement, such as e.g. for measuring pH-value in aqueous media [1], wherein the use of such ion-sensitive sensors has increased somewhat in the last years in industry by applying EIS structures. Ion-sensitive sensors having an EIS structure are suitable for miniaturizing a measuring system especially through the relatively cost effective manufacture of integrated systems and are especially superior to the application of conventional glass electrodes. Thus, instead of the inner buffer with glass interface, a well manageable Si/SiO2 combination is utilized. A further advantage of the sensors based on EIS structures is that such can be produced without the material, glass, whereby the requirement is met that in the case of some applications a glass breakage risk is not desired, respectively necessarily must be avoided.
Expressed generally, an ISFET sensor can be viewed as an impedance converter integrated in the sensor, which represents a still higher impedance of the EIS structure than that of a glass electrode, wherein, however, the measured variable is converted directly on-site into an easily and precisely measurable, low impedance signal. Through the application of the operating mode (“constant charge”), an ISFET sensor is, furthermore, able to suppress signal disturbances due to environmental light, respectively ambient light, relatively well.
A capacitively readable EIS sensor without FET structure can, for example, be so constructed that no topology edges disturb the surface, respectively the surface character, so that the risk of action of chemically aggressive media on possible topology edges can be lessened. The read-out of such an EIS sensor occurs, for example, via a capacitance measurement.
The LAPS sensors (LAPS=Light Addressable Potentiometric Sensor) make use of sensitivity resolved laterally along the surface and are suited, for example, for biochemical systems, wherein the region to be detected can be selectively irradiated with light [13].
Since the previously achieved chemical and electrical, long term stability of ion-sensitive sensors based on EIS structures is still not sufficient for use in process measurements technology, respectively corresponding ion-sensitive sensors are extremely complex and therewith expensive to manufacture, previously no lasting introduction of EIS-based pH-measuring could occur in industrial process measurements technology and in environmental monitoring.